12. Storage Practices

By Tom Phillips

The majority of wheat harvested in the Great Plains is managed through a net- work of grain elevators from which it is either processed, exported, or stored. Only about 20 percent of the crop is stored on farm. Grain is in its best condition at the point of harvest, but the impacts of harvesting, moving, and storing will ultimately lower the quality and marketability of grain. The objective of stored grain management is to slow or deter this loss of quality so that grain can attain its highest potential market value.

IPM and Safe Grain Storage

An integrated pest management (IPM) approach can be adopted for storage of wheat just as it is being adopted for production. IPM of stored wheat requires the grain elevator, or producer with on-farm storage, to be knowledgeable about the pests of stored grain, the conditions of the grain, and the storage structures. The key to safe storage of grain is to store clean, dry grain in insect-free structures and maintain grain temperatures as cool as possible. Such ideal conditions are difficult to achieve in some Great Plain regions, such as Oklahoma. However, we are fortunate that the climate in Oklahoma facilitates proper drying in the field before harvest in most years, since wheat moisture content should be below 13 percent for safe storage. The practice of stored grain IPM integrates the use of preventive measures to avoid pest problems by monitoring insect levels and grain condition, cooling grain with aeration to slow infestations, and using effective chemical pesticides when needed to avoid economic impacts due to infestation.

Figure 12.1 Small grain storage bins in Fort Collins, Colorado.

Small grain storage bins in Fort Collins, Colorado

Figure 12.2 (a) The lesser grain borer.

the lesser grain borer

Figure 12.2 (b) Damage to kernels caused by the lesser grain borer’s feeding habits.

Damage to kernels caused by the lesser grain borer’s feeding habit

Figure 12.3 The rice weevil.

the rice weevil

Figure 12.4 The rusty grain beetle.

the rusty grain borer

Storage Pests

Insect pests infesting stored wheat fall into one of two categories based on their feeding habits- internal or external feeders. Internal feeders are those whose larvae feed on the inside of grain kernels and bore holes through grain, such as the lesser grain borer (Figure 12.2 a & b) and the rice weevil (Figure 12.3). Because these two species contribute to the grading factor of “insect damaged kernels,” or IDK, they are considered serious economic pests. In fact, the lesser grain borer is the most serious pest of stored wheat in the Great Plains.

On the other hand, external feeders are unable to penetrate the seed coat, either as adults or larvae, and make their living by feeding on broken kernels, grain dust, fungi, and most forms of milled grain products (such as flour and feed). Although external feeders do not contribute to IDK, grain will be designated “infested” if two or more live specimens are found in a sample when graded (discovery of dead insects does not result in the designation “infested”). One of the most common external grain feeders in the Great Plains region is the rusty grain beetle (Figure 12.4), which is relatively small compared to the next most common insect, the red flour beetle (Figure 12.5). Other external grain feeders of importance include the sawtoothed grain beetle (Figure 12.6), the hairy fungus beetle (Figure 12.7), and the Indianmeal moth (Figure 12.8). The worm-like larvae of the Indianmeal moth can cause problems when high populations deposit large amounts of silk on the top of grain, which in turn blocks aeration and causes grain heating and molding.

Figure12.6 The sawtoothed grain beetle.

Howover; not all in""'t'in ain

may be attaGktxl ard kiu..l by various spo:ies of predatorsand para•lte• that rroy also OGGur in the gain. Those rntural enemie.ac.tually hdp to regulat<. and sanetime• reduc.<. populations dins«.tIf anunlmowninseot is found ingrain the­

men should bedirootal to your localcooperativee><lension oftlcdor Identification

Prol:>lemswith non-ins«.tpe<t•canalso =but most areno1 uprevalen or

persittentu those with insects. Somespede< of fungi (mold) anbulld moioture pinand may cause problems with thepro:luotion of mycotcodns.

the hot,dry c.on!!Uons in most Great Plains wheat storage faGllltie< preclude seriou•

problem$ with fungi. Inaddition. vertebrate pests suc.h as bird <.


taminaie1t with hair, fooes,and urire.

Figure 12.7(left) The hairy f1mgus beetle.

Figure 12.8


The brlianmeal moth (adultand

larvae R'3'S).

Sanitation and Structural Maintenance

Maintaining storage structures that are clean and secure is the cornerstone of
pest prevention in stored grain IPM. The primary source of new infestation is insect breeding in grain from a previous storage season. Storage pests must have dried grain or grain debris in which to live and reproduce. Spilled grain, grain trash, or carryover grain left in a bin can serve as breeding material for insects and a source of infestation on new grain. Since stored grain pests do not breed on wheat growing in the field,
they do not enter storage facilities with the new crop when it is stored. However, most grain pests are good fliers and can move from bin to bin and farm to farm, and enter grain bins or silos through any number of small openings.
Harvesting and transportation equipment must be cleaned prior to harvest be- cause residues of old grain and grain pests can contaminate the new grain before reaching the bin. All empty bins, silos, and flat storage structures should be com- pletely free of old grain before the new crop is stored. Prior to storage, grain managers need to go inside round steel bins and flat storages and thoroughly sweep and remove as much old grain and debris from floors and side-walls as possible. Old grain in con- crete silos should also be removed and structures similarly cleaned if access is avail- able.
Since it is impossible to clean every last bit of grain from empty bins, additional protection can be achieved by spraying the inside surfaces of bins with an appropriate residual insecticide. If substantial carryover grain exists (that from a previous crop),
it should be consolidated into one or more bins and monitored carefully or treated appropriately. Never store new grain on top of old. Loose grain around the outside of bins, and on floors in the basements and galleries of concrete houses, should be
cleaned immediately after it is spilled. Volunteer wheat and other vegetation growing around bins should be removed because it can harbor insects and rodents.
All grain-moving equipment, including bin walls, roofs, doors, and hatches, should be in a good state of repair. Once a bin is full it is extremely difficult to per- form repairs on load-out augers or conveyors. Holes in roofs leave grain susceptible to rain water, and wet grain can sour readily and support high levels of mold and insect infestation. Large openings from inoperable doors or hatches can allow entry of more immigrating insects than would typically occur through smaller openings. Addition- ally, closures should be made as tight as possible so they can be properly sealed in the event of a fumigation treatment to the structure.


Monitoring, also referred to as “scouting” when discussing field crop production, is very important in IPM because it is the only way to gain valuable information that will facilitate control or management decisions. Once new grain is safely stored in
a relatively insect-free structure, it must be monitored regularly for pest problems,

or the potential for pest problems. Three factors that should be monitored in stored grain are grain temperature, grain quality, and insect density.

Grain Temperature

Temperature is important because cool grain, that which is less than 60"F (16°C), will prevent excessive growth of insect populations, while increasingly higher tem­ peratures will allow populations to flourish. Large commercial storage structures­ whether flat storage buildings, round steel bins, or concrete silos-should be equipped with temperature monitoring cables that can provide the manager with a profile of grain temperatures throughout a grain mass. Temperature information is transmit­ ted electronically from each thermocouple to a reading device that allows the user to record temperatures and thus keep records over time.

Whether grain temperatures are high or low at a given time, it is desirable to main­ tain fairly uniform temperatures throughout a grain mass and to observe only small changes from week to week. When a thermocouple, or two or more closely situated thermocouples, read five or more degrees higher than the others in a bin for two or more consecutive weeks, the manager has reason to suspect grain heating from some pest or moisture problem. Such grain temperature "warning signs" suggest that man­ agers should turn the grain or treat the grain to break up the hot spot(s).

It is also important to monitor the change in grain temperature during the course of aeration cooling or through passive seasonal cooling or warming. In the absence of temperature cables, a grain manager can check grain temperatures directly by manual inspection. A protected mercury thermometer mounted on a probe can be inserted into grain, or an inspector can determine temperature in the top three to four feet of the mass by simply touching the grain with the hand and arm.

Grain Quality and Insect Density

Direct inspection of grain samples, as with remote sensing of grain temperature, is an important monitoring activity because additional information for pest manage­ ment decision-making can be gathered. Grain samples should be taken from several locations throughout a storage structure at monthly intervals to count the number and species of insects, if any are present, and to assess grain quality. Grain samples can be collected from standing grain using either a deep cup probe sampler or a long, spear­ like grain trier. A kilogram or more (over two pounds) of grain from each sampling point is adequate for making an assessment; and as many samples as possible should

be taken from each structure.

Steel bins and flat storages allow access to the top of a grain mass where samples can be taken. Take appropriate safety measures when entering a confined space such as a grain bin; do not work alone. Concrete silos present problems for collecting grain samples, although samples from the bottom of the silo can be taken in most facilities by accessing grain at the hopper bottom. Some silos are full enough to allow for deep cup or probe samples to be taken from the top. Once obtained, a grain sample should be sieved thoroughly to remove any insects. Any insects found should be identified,

counted, and recorded to observe monthly trends. Presence of external feeding insects in dry, otherwise sound wheat, are not cause for serious alarm. However, the presence of the grain-damaging internal feeders should warrant further inspection.

Insects can also be monitored using grain probe traps. Probe traps should be inserted into the top of a grain mass and checked weekly or biweekly. Probe traps
do not use attractants, but simply capture insects that are moving through the grain. Capture will depend on the species of insect, the total number of insects in the grain, and the grain temperature. Numbers of insects caught in grain probe traps can not be converted into insect densities, such as the number of insects per bushel that can be determined from grain samples, but probe-trapped insects can inform the manager about the species present and population trends over time. It is possible to capture hundreds of non-grain damaging, external feeding insects in probe traps over a one to two week period in the summer without need for a control action, provided that other indicators suggest the grain is in good condition. However, the capture of one or more grain damaging insects, particularly the lesser grain borer, should alert the manager toward further investigation and possible control.
Grain quality should be assessed from samples with the best methods available. Dampness, off-odor, or moldy appearance are immediate signs of wet, deteriorating grain. Other quality factors include moisture content, test weight, dockage, and pres- ence of IDK. Test weight and dockage information from in-bin grain samples will aid the manager in marketing decisions, while the presence of high moisture grain and any level of IDK can signal a pest problem. Consistent findings of grain damaging insects (borers and weevils) and IDK should be a trigger for a control action, such as fumigation.


Aeration fans should be used to cool grain with outside air once the air tempera- ture drops considerably below that of the grain temperature. Cooling grain will not kill insects outright, but it will substantially slow their growth and development. For example, grain insects held at temperatures below 65°F (18°C)will feed and grow very little, and their eggs will take months to hatch.
Aeration fans at the base of bins should be directed to blow out and draw cool air down (suction mode) through the grain mass from vents in the roof. Intake aeration from the bottom, or blowing mode, is appropriate only if an adequate number of roof vents, some equipped with exhaust fans, are in place and operating to carry mois-
ture out of the bin. Air flow rates of 0.1 to 0.5 cubic feet per minute per bushel (cfm/ bu) are recommended for wheat at normal moisture contents and can be achieved by matching fan motor power with the depth of grain being aerated (Table 12.1).
In some Great Plains regions, the most effective grain cooling by aeration occurs
in the late summer and early fall, when nighttime temperatures fall below 60°F (16°C). At this time fans should only be left on at night and not during the day when air tem- peratures may still be in the high 80s and 90s. Aeration fans need to run for several consecutive nights in order to cause a significant lowering of grain temperature. The exact number of accumulative hours will depend on the amount of grain being aer- ated, the depth of the grain in the bin, airflow rates of the fans, and the difference between grain and air temperatures. Automatic aeration controllers can be installed

on fan motors that will turn fans on when the outdoor temperature goes below a given

Table 12.1

Approximate aeration fan horsepower required per 100 bushels of wheat.1

Grain Depth (ft)































































1Total horsepower for all fans on a single bin.

2 Air flow rate in cubic feet per minute per bushel of wheat.

set point, and will turn the fans off when the outdoor temperature exceeds the set point. Controllers can also record the number of hours fans are turned on so that the progress of the cooling front can be estimated. Automatic controllers may also allow managers to begin grain cooling earlier in the summer than they might otherwise begin if they were controlling fans manually during the fall.

Winter aeration of grain in the cool months of November, December, or January can be performed both night and day for several days in order to achieve a very cool and safe storage temperature. Such cool temperatures can sometimes be maintained well into the next storage season. Implementation of proper grain cooling with aera­ tion requires several economic and technical considerations by the manager. Consult OSU publications E-912, F-7180, and No. 1100 for details of proper grain aeration.

Chemical Prevention, Control, and Alternatives

The practice of IPM instructs the manager that the cost of controls, such as ap­ plication of pesticides, and the cost of pest preventive measures, such as the electrical cost of cooling grain with aeration, must be outweighed by the return in value of the commodity that is protected In stored grain it is sometimes difficult to determine

if an expenditure for pest management will be cost-effective, especially if it is made

several months before the grain is sold.

Residual Insecticides

Residual insecticide sprays for treating grain bins and sprays for direct treatment
of new grain while it is loaded into bins are available. For information on insecticides registered for direct application to grain, refer to Kansas State University’s “Stored Grain Management Options” found at www.entomology.ksu.edu.
The use of residual grain protectants is typically limited to high value raw com- modities that need protection during several months of storage and for which it is
cost-effective to use such material. The decision to use a residual insecticide on stored wheat is an important one that will require information about costs, benefits, and
risks. There are very few cases in the Great Plains in which the use of residual insecti- cides are warranted for direct application to stored wheat.
However, residual sprays can be effective for treating empty grain storage struc- tures. As previously discussed, steel bins, silos, and flat storage structures should
be emptied and thoroughly cleaned prior to treatment. The residual spray serves to kill insects that are hiding in the structure and those that enter the bin from outside. Thus, a spray to the inside surface of a grain bin should act like a protective envelope around the grain.


Fumigation is used to kill insects and stop infestations when insect populations
reach undesirably high levels or grain damage is unacceptable. For example, a grain manager may choose to fumigate if grain samples reveal the presence of IDK and probe traps, or other monitoring activities, detect lesser grain borers. Fumigation in this case will stop infestation and grain degradation from getting worse and will allow the manager to then either blend the damaged grain to reduce total IDK or take other action. However, many commercial grain elevators will fumigate grain just before it is sold, whether grain-damaging insects or IDK are present or not, to ensure that no live insects are present when the grain is evaluated by the buyer. Such fumigations can be cost-effective because discounts for infested grain are avoided. Similarly, fumigation
at the end of the summer will suppress insect populations in grain stored through the fall and winter. When fumigations are effectively conducted in August or September and are followed by fall and winter aeration, pest populations can be greatly reduced.

Fumigants registered for use on stored wheat are phosphine gas, generated from either aluminum or magnesium phosphide, and methyl bromide. Methyl bromide is expensive, difficult to use properly on raw grain, and is not recommended for stored grain. The fumigant used nearly universally for stored wheat is aluminum phosphide in the form of pellets or tablets. It is sold under the trade names of Weevilcide, Fumi- toxin, or Phostoxin. These materials are potentially very dangerous if improperly used or handled. Strict safety guidelines are in place to protect those applying phosphine and those working in areas where phosphine is being used. Fumigations must be conducted by licensed applicators who have received specific training in grain fumi- gation and fumigation safety.

Phopshine gas requires up to a week to kill eggs, larvae, pupae, and adults of grain insect pests. Effective fumigation treatment requires:

1. A sufficient number of pellets or tablets distributed throughout stored grain to generate an adequate level of gas.

2. Optimal grain moisture and temperature for gas generation.

3. Minimization ofleaks in a structure so that gas can be held on the grain for the required time.

Aluminum phosphide generates phosphine gas after it is exposed to moisture in the air. In concrete houses, aluminum phosphide pellets can be added to infested grain as it is being transferred from one location to another. When infested grain cannot easily be moved or turned, such as in large steel bins or flat storages, pellets or

tablets should be probed as deeply into the mass as possible, and also distributed on or near the top surface. Phosphine gas is as light as air and moves easily through grain and out ofleaks in structures just as smoke would move. Since small amounts of gas are being released from each pellet or tablet, it is important that these point sources be well-distributed throughout a grain mass. However, since the gas has a tendency to move passively upward with convection currents, a larger distribution of pellets in the bottom of a mass is recommended.

For steel bins or flat storages a re-circulation system known as closed-loop fumiga­ tion (CLF) should be employed. CLF utilizes a light-duty blower fan with an array of PVC pipes to draw phosphine gas from the top headspace and re-circulate it down to the bottom aeration system of the bin. Gas that returns to the bottom of the bin can then rise through the grain mass and achieve a uniform distribution.

Leaky bins and silos contribute to most of the fumigation failures in stored wheat. Many steel bins and flat storages have numerous leaks and would not meet the mini­ mum levels of gas-tightness for a good fumigation treatment. Concrete silos have potential for being low-leak structures, but they can easily lose substantial amounts of gas before kill is achieved if inter-vents between silos and outside vents are not sealed during fumigation. Phosphine fumigation is a technical operation that should be un­ dertaken only by skilled professionals. Ineffective fumigation can lead to poor insect control and pest resistance to phosphine.

Many other materials and practices are registered for use in killing stored grain pests, but they are not considered here because they are either not widely used or are considered ineffective or inappropriate for most stored wheat situations in the Great Plains region. Research is ongoing worldwide to find safe and effective alternatives to dangerous chemical pesticides and to develop efficient and cost-effective pest manage­ ment methods. Much ofiPM in stored grain involves application of common sense once the manager has a good understanding of the pests and the commodity being managed. Grain managers should maintain a disciplined watch over their grain while it is in storage because it represents a substantial financial investment for them and their customers.


Oklahoma State University Stored Products and Research Education Center



OSU Stored Product Management—Circular Number E-912.

Purdue University Post Harvest Grain Quality:


USDA ARS Stored Product Insect Research Unit: